Biomineral Formation

Sea Urchin Teeth

Because of the fundamental nature of research on the physical aspects of biominerals, their formation mechanisms, the potential for future bio-inspired materials synthesis, and the aesthetic beauty of biomineral structures, scientists of all ages are interested in biomineralization. Biominerals include mollusk shells, the skeletons of sea urchins, corals, and mammals.

Biomineral formation mechanisms fascinate physicists, materials scientists, and chemists because they result in materials more robust than their components. The beauty of biominerals begins with amorphous—that is, non-crystalline—solids that transition under biological control into crystalline structures with curved morphologies, very different from the familiar faceted crystal shapes. Biomineral crystals form by aggregation of amorphous nanoparticles, and subsequent crystallization. This mechanism is very different from the usual atom-by-atom crystal growth.

Biomineral formation can be seen in sea urchin teeth. Image 1 shows the intricate structures by which sea urchin teeth fill space and resist fracture. Sea urchin teeth have many plates of crystalline calcite, all co-oriented. Until recently, a topological connection between these crystalline calcite plates had never been identified. Image 2 shows the newly discovered calcite mineral bridges, which provide such connection, hence all plates are topologically one connected crystal.

Image Credit: Pupa Gilbert, University of Wisconsin-Madison (2011)

Image 1

This false-color scanning electron microscope (SEM) micrograph shows the forming end of a tooth from the Florida sea urchin Arbacia punctulata. Each tooth element is highlighted by a different color, and is a single crystal of calcite (CaCO3). Unlike geologic crystals, these biomineral crystals do not have flat surfaces and sharp edges, but show curved surfaces.

Image Credit: Pupa Gilbert, University of Wisconsin-Madison (2011)

Image 2

This SEM false-color micrograph of Eucidaris tribuloides shows the recently discovered calcite mineral bridges (elongated orange structures) that connect the tooth plates (red and black areas under the orange bridges). The bridges are resorbed before the tooth erupts.